Bizarre this:-
 
X3 has been solid so far, really impressed.  It has just crashed for the first time, and I wasn't even using it..
 
Had been listening back to a piece written and started in X3a.  Left it for a while, and then was doing something in Internet Explorer with Sonar still on in the background, but not playing.  I copied some text in IE and as I went to paste it Sonar crashed, and sent of problem report.  I didn't even touch it, honest..
 

CWBRN-20135 Problem Report

Would you have the default not to share audio driver with other software?
 
Just popped up as an idea it could be related...

I think you may be a genius, Sir.
 
I have always set "Share audio driver" to "yes", but when I looked now, it is unchecked..
 
Could that be it?  Perhaps that preference was not carried over to X3.
 
Thank you so much.  I may now try to reproduce it.